A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus generally includes a motion control system. This motion control system includes position detectors configured to detect a position of, for example, a substrate table of the lithographic apparatus in at least one plane, i.e. in at least two dimensions. Further the motion control system includes a controller which is constructed to drive actuators in dependency on an output signal provided by the position detectors. The motion control system forms part of a feed forward and/or feed back control system ensuring that the table is in a correct position within a certain tolerance range. A position of the table is each time detected by the position detectors, and a difference between the detected position and a desired position is reduced by an appropriate action of the controller.
In a current lithographic apparatus, a desired accuracy for the table is in the order of magnitude of nanometers. Hence, it is desired that the position detector be able to achieve such a high accuracy. Furthermore, requirements on the position detector are also high in that the range within which the position detector generally operates is relatively large (e.g. up to 0.5 m for the substrate table). The known position detectors include interferometers. Interferometers, however, may be expensive.
A further type of position detector is an optical encoder. Such an optical encoder includes a light or radiation source, a grating and a detector. By moving the grating with respect to the radiation source and the detector, changes occur in the radiation pattern as received by the detector due to e.g. reflection or transmission changes. The grating is thus included in an optical path from the radiation source to the detector. If the grating is moved, the pattern as received by the detector changes. From these changes, displacement of the grating with respect to the radiation source and detector can be calculated. From a known starting position and a calculated displacement, an absolute position can be calculated. Since encoders are not always absolute sensors, it is desirable to measure a zero position by an additional sensor.
A specific type of optical encoder includes a diffraction type encoder including a radiation source, a first grating, a second grating, and a detector. The second grating is moveable with respect to the first grating, and the detector is arranged to detect a diffracted beam of the radiation beam as diffracted on the first and the second grating. One of the gratings is mechanically connected to the substrate table, the other one of the gratings is mechanically connected to a reference frame of the lithographic apparatus. A movement of the substrate table causes a movement of the first grating with respect to the second grating and in operation causes a change in the diffracted beam.
Measurements of the substrate table in z-direction, that is to say a direction perpendicular to the x-y plane of movement of the substrate table, is also done with the aid of encoders. Also, such a diffraction type encoder is not an absolute sensor, and for measuring and defining a zero position an additional sensor may be used.
Patterning device stage position measurement in z-direction is done with capacitive sensors. These are absolute sensors. However, present capacity sensors are not able to reach sufficient accuracy. In particular, a lower noise and vacuum compatibility is desirable. Using a 2 D encoder head may provide an improvement in this matter, but may lack an absolute zero position in z-direction.